Assessing Long-Term Postseismic Transients From GPS Time Series in Southern California

In order to gain insight into long-term plate boundary motion and to shed light on geodetic-based fault slip rates and the seismic hazards they inform, we apply a forward modeling strategy to identify and reduce the short- and long-term effects of viscoelastic postseismic deformation on modern GPS observations following large magnitude earthquakes in Southern California. We assess ongoing postseismic deformation in the southwestern United States by analyzing all magnitude ≥M_w6.0 earthquakes that have occurred there and in Baja California and Sonora, Mexico, since year 1800, finding that ongoing postseismic displacements from 12 events are potentially contributing to the modern day deformation field in Southern California. With a forward modeling step, we calculate postseismic displacements associated with these 12 events using a reference model consisting of a layered, laterally homogeneous, viscoelastic Earth structure; these displacements are then subtracted from processed horizontal GPS coordinate time series data to produce a postseismic-reduced data set. In order to quantify the success of this forward modeling in reducing the postseismic signal, we estimate parameters representing logarithmic decay associated with the 2010 M_w7.2 El Mayor-Cucapah earthquake using two different time series analysis methods. Variance reduction indicates we were able to reduce postseismic deformation in this test case by up to 60%. Anomaly maps produced using our assessment of deformation around the El Mayor-Cucapah event highlight hot spots in which secondary processes may be occurring or where a more complex viscosity structure may be necessary.